High strength, high ductility titanium-alloy and process for producing the same

ABSTRACT

A high strength, high ductility titanium alloy comprising O, N and Fe as strengthening elements and the balance substantially Ti, the contents of the strengthening elements satisfying the following relationships (1) to (3): 
     (1) from 0.9 to 2.3% by weight of Fe, 
     (2) up to 0.05% by weight of N, and 
     (3) an oxygen equivalent value Q, which is defined by the formula mentioned below, of 0.34 to 1.00 
     
         Q=[O]+2.77[N]+0.1[Fe] 
    
     wherein [O] is an oxygen content (% by weight), [N] is a nitrogen content (% by weight) and [Fe] is an iron content (% by weight), the titanium alloy having a tensile strength of at least 700 MPa and an elongation of at least 15%. Part of Fe may be replaced with Cr and/or Ni. Fe, Cr and Ni may be introduced from a carbon steel or stainless steel, or they may be introduced from sponge titanium containing these elements.

TECHNICAL FIELD

The present invention relates to a high strength, high ductilitytitanium alloy and a process for producing the same. The presentinvention relates, in more detail, to a high strength, high ductilitytitanium alloy containing no alloying elements which increase theproduction cost, such as Al, V and Mo, and having a tensile strength ashigh as at least 700 MPa, preferably at least 850 MPa, particularlypreferably at least 900 MPa and an elongation as high as at least 15%,preferably at least 20%, and a process for producing the same.

BACKGROUND ART

(α+β)-alloys and β-alloys containing Al, V, Zr, Sn, Cr, Mo, and the likehave heretofore been known as high strength titanium alloys. In general,these conventional alloys have a tensile strength of at least 900 MPa,and there are few titanium alloys having a strength level between thatof pure titanium and that of the conventional alloys, namely from about700 to 900 MPa.

For example, Ti--6Al--4V alloy is a typical alloy of the (α+β)-alloys,and has a tensile strength of 850 to 1,000 MPa and an elongation of 10to 15% in an annealed state. There is Ti--3Al--2.5V alloy which has astrength level lower than the alloy mentioned above, and which has atensile strength of 700 to 800 MPa and is excellent in ductility.

However, since these alloys contain V which is a high cost alloyingelement, they have the disadvantage that their cost is high.

Accordingly, the alloys mentioned below have been proposed in which V, ahigh cost alloying element, is replaced with Fe, a low cost element:Ti--5Al--2.5Fe alloy ("Titanium Science and Technology," DeutcheGesellshaft fur Metallkunde E.V. p1335 (1984)), andTi--6Al--1.7Fe--0.1Si alloy and Ti--6.5Al--1.3Fe alloy (AdvancedMaterial & Processes, p43 (1993)).

However, the above alloys which have been proposed contain a largeamount of Al, and have high strength and low ductility at hightemperature. The alloys have, therefore, poor hot workability comparedwith pure Ti. These alloys have the problem that the hot working cost isstill high though the raw material cost is lowered by replacing V withFe.

Accordingly, an alloy has been proposed which contains neither Al nor Vand which utilizes O (oxygen) and N (nitrogen) as interstitialstrengthening elements. For example, Japanese Patent Kokai PublicationNo. 61-159563 discloses a process for producing a pure Ti forgedmaterial having a tensile strength at the level of 80 kgf/mm² class andan elongation of at least 20% which process comprises rough forging athigh temperature including upsetting forging, finish forging, and heattreating at temperature of 500 to 700° C. for up to 60 minutes. Theprocess, however, requires complicated forging such as upsetting forgingand heavy deformation, and it cannot be adopted in general.

Japanese Patent Kokai Publication No. 1-252747 discloses a high strengthtitanium alloy excellent in ductility which requires no specificforming, and which can be formed into products having various shapessuch as sheets and rods by conventional rolling. The titanium alloydisclosed herein contains O, N and Fe as strengthening elements. Thecontents of these strengthening elements are defined as follows: the Fecontent is from 0.1 to 0.8% by weight, and the oxygen equivalent valueQ, which is defined to be equal to [O]+2.77[N]+0.1[Fe], is from 0.35 to1.0. The N content is defined to be practically at least 0.05% by weightas disclosed in examples, and the titanium alloy is made to have finemicrostructure in the (α+β) dual and equiaxed phase or lamellar layers.As a result, the titanium alloy has a tensile strength of at least 65kgf/mm².

The disclosed titanium alloy attains a tensile strength of at least 65kgf/mm² and an elongation of at least 20% by solid solutionstrengthening with O and N, and by microstructural grain refiningeffects obtained by utilizing an Fe content higher than that of puretitanium, and it attains a tensile strength of at least 85 kgf/mm²particularly when Q≧0.6.

However, as shown in FIGS. 1 and 2 in the patent publication, thetitanium alloy has a tensile strength of up to 95 kgf/mm² when Q≦0.8,though it has an elongation of at least 15%. Moreover, though thetitanium alloy has a tensile strength as high as from 95 to 115 kgf/mm²when Q=0.8 to 1.0, it has an elongation as low as up to 15%.

As described above, the titanium alloy does not always have both a highstrength and a high ductility at the same time. Accordingly, a furtherdevelopment of a titanium alloy having both a high strength and a highductility is desired.

Furthermore, although the alloy requires a N content as high as at least0.05% by weight, the addition of such a large amount of N is extremelydifficult in the production of the alloy by melting. Control of theaddition amount is also difficult.

That is, since melting titanium is conducted in vacuum or in an inertgas atmosphere at low pressure, introducing nitrogen using a nitrogengas is almost impossible during melting. Nitrogen, therefore, must beintroduced in the form of a nitrogen-containing solid. To avoid acontamination with impurities which exert adverse effects on theproperties of titanium, the addition of nitrogen-containing titanium ispreferred. To obtain such a high nitrogen content as mentioned above, atechnique such as addition of titanium containing a large amount ofnitrogen becomes necessary. As a result, a compound such as TiN having avery high melting point of 3,290° C., and likely to form an undissolvedportion, may form. Such undissolved TiN, etc. may remain asnitrogen-rich inclusions in the titanium alloy, and it may form a fataldefect such as the starting point of a fatigue failure. Moreover, sincenitrogen is a gas component, the introduced nitrogen tends to evaporateeven when the nitrogen is introduced in the form of anitrogen-containing solid, and control of the nitrogen content isdifficult.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a titanium alloy havinga still higher strength and a still higher ductility compared with theconventional alloys mentioned above while the content of nitrogen whichis difficult to add is decreased.

According to a first aspect of the present invention, the object isachieved by a high strength, high ductility titanium alloy comprising O,N and Fe as strengthening elements and the balance substantially Ti, thecontents of the strengthening elements satisfying the followingrelationships (1) to (3):

(1) from 0.9 to 2.3% by weight of Fe,

(2) up to 0.05% by weight of N, and

(3) an oxygen equivalent value Q, which is defined by the formulamentioned below, of 0.34 to 1.00

    Q=[O]+2.77[N]+0.1[Fe]

wherein [O] is an oxygen content (% by weight), [N] is a nitrogencontent (% by weight) and [Fe] is an iron content (% by weight), thetitanium alloy having a tensile strength of at least 700 MPa and anelongation of at least 15%.

According to a second aspect of the present invention, the object isalso achieved by a high strength, high ductility titanium alloycomprising O, N, Fe and at least one element selected from Cr and Ni asstrengthening elements and the balance consisting substantially of Ti,the contents of the strengthening elements satisfying the followingrelationships (1) to (6):

(1) from 0.9 to 2.3% by weight of the total amount of Fe, Cr and Ni,

(2) at least 0.4% by weight of Fe,

(3) up to 0.25% by weight of Cr,

(4) up to 0.25% by weight of Ni,

(5) up to 0.05% by weight of N, and

(6) an oxygen equivalent value Q, which is defined by the formulamentioned below, of 0.34 to 1.00

    Q=[O]+2.77[N]+0.1{[Fe]+[Cr]+[Ni]}

wherein [O] is an oxygen content (% by weight), [N] is a nitrogencontent (% by weight), [Fe] is an iron content (% by weight), [Cr] is aCr content (% by weight) and [Ni] is a Ni content (% by weight), thetitanium alloy having a tensile strength of at least 700 MPa and anelongation of at least 15%.

According to a first viewpoint of the first or second aspect of thepresent invention, a high strength, high ductility titanium alloy whichhas the oxygen equivalent value Q of 0.34 to 0.68, a tensile strength of700 to 900 MPa and an elongation of at least 20% is provided.

According to a second viewpoint of the first or second aspect of thepresent invention, a high strength, high ductility titanium alloy whichhas the oxygen equivalent value Q of 0.50 to 1.00, a tensile strength ofat least 850 MPa and an elongation of at least 15% is provided.

According to a preferred mode based on the second viewpoint of the firstor second aspect of the present invention, a high strength, highductility titanium alloy which has the oxygen equivalent value Q ofgreater than 0.68 to 1.00 and a tensile strength exceeding 900 MPa isprovided.

Furthermore, a third aspect of the present invention is a process forproducing a high strength, high ductility titanium alloy according tothe first or second aspect of the present invention which processcomprises charging and melting at least one steel selected from carbonsteels and stainless steels during the production of the titanium alloyby melting, so that Fe, or at least part of Fe, Cr and Ni as thestrengthening elements is introduced from the steel.

A fourth aspect of the present invention is a process for producing thehigh strength, high ductility titanium alloy according to the first orsecond aspect of the present invention which process comprises producingsponge titanium by the use of a vessel containing Fe, or at least oneelement selected from Fe, Cr and Ni in the step for producing spongetitanium, so that the sponge titanium contains Fe or the at least oneelement selected from Fe, Cr and Ni which has been transferred therefromand has invaded, and supplying the sponge titanium as at least part ofthe supply raw materials for Fe, or for the at least one elementselected from Fe, Cr and Ni, as the strengthening element during theproduction of the titanium alloy by melting.

Although nitrogen which is an interstitial solid-solution elementdissolved in the α-phase to solid-solution strengthen the alloy, controlof the amount thereof necessary for strengthening during melting by VAR(vacuum arc melting) or the like is difficult. Moreover, when thecontent is excessive, the ductility is unpreferably lowered. In thepresent invention, therefore, the addition and the content control ofnitrogen are made easy by decreasing the N content. Since nitrogen maybe added in a decreased amount, N-rich inclusions in the raw materialsfor melting are decreased to such an extent that they can be made todisappear by VAR.

However, when the addition amount of N is decreased, the degree ofstrengthening the titanium alloy with N is also decreased. To ensure thestrength, it is satisfactory to supplement a decrease in the amount of Nwith O or Fe which is a strengthening element. However, an increase inthe amount of O lowers ductility. An increase in the amount of Fesimilarly lowers the ductility. The latter instance is disclosed, forexample, in test Nos. 9 and 10 of Table 3 in Japanese Patent KokaiPublication No. 1-252747.

As a result of conducting various experiments for the purpose ofimproving the strength as well as the ductility, the present inventorshave discovered that an increase in the amount of Fe lowers theductility when the N content is at least 0.055% by weight, and that anincrease in Fe, therefore, does not lower the ductility but improves thestrength when the N content is made less than 0.055% by weight,particularly when it is made less than 0.050% by weight. That is, thestrength and the ductility are simultaneously improved by adjusting theN content to up to 0.05% by weight and the Fe content to at least 0.9%by weight.

The reasons for the effect described above are described below.

Since Fe is a β-phase-stabilizing element, an increase in the amount ofFe increases the amount of the β-phase, and as a result the amount ofthe α-phase decreases. Consequently, N which is an α-phase-stabilizingelement is enriched in the α-phase which has decreased in amount. Whenthe N content exceeds 0.05% by weight, a Ti₂ N superlattice phase tendsto precipitate in the α-phase due to the enrichment, and theprecipitates lower the ductility. The restriction of the N content to0.05% by weight makes such a precipitation phase difficult to form, andan increase in the amount of Fe improves the strength.

When O exists in an excessively large amount, the superlattice phases ofTi₃ O and Ti₂ O are formed. However, the amount of O necessary forforming these superlattice phases is particularly large compared withthat of N, and does not matter at all in the scope of the presentinvention.

According to the present invention, a titanium alloy attains a tensilestrength of at least 700 MPa and an elongation of at least 15%. When atitanium alloy is solid-solution strengthened by simply increasing theamounts of O and N, the ductility is lowered, though the strength isincreased. In the present invention, the N content is decreased to up to0.05% by weight and then the amount of Fe is increased to at least 0.9%by weight, whereby the amount of the β-phase having good ductility isincreased and good ductility of the alloy is ensured. At the same time,the contents of O, N and Fe which are strengthening elements areadjusted so that the oxygen equivalent value Q satisfies therelationship Q=0.34 to 1.00. As a result, the titanium alloy attains atensile strength of at least 700 MPa and an elongation of at least 15%.The oxygen equivalent value Q herein is defined by the followingformula:

    Q=[O]+2.77[N]+0.1[Fe]

wherein [O] is an oxygen content (% by weight), [N] is a nitrogencontent (% by weight) and [Fe] is an iron content (% by weight).

Especially according to the first viewpoint of the present invention,when the Q value is made to fall in a range of 0.34 to 0.68, a highstrength titanium alloy particularly excellent in ductility is obtainedwhich has a tensile strength of 700 to 900 MPa and an elongation of atleast 20%. To ensure a tensile strength of at least 700 MPa, the Q valueis required to be at least 0.34. To ensure an elongation of at least20%, the Q value is required to be up to 0.68.

Furthermore, according to the second viewpoint of the present invention,when the Q value is made to fall in a range of at least 0.50 to 1.00, atitanium alloy is obtained which has a tensile strength of at least 850MPa and an elongation of at least 15%, that is, which is ensured to havea still higher strength and a good ductility. To ensure a tensilestrength of at least 850 MPa, the Q value is required to be at least0.50. To ensure an elongation of at least 15%, the Q value is requiredto be up to 1.00.

According to a preferred mode based on the second viewpoint of thepresent invention, when the Q value is made to fall in a range ofgreater than 0.68 to 1.00, a titanium alloy is obtained which has atensile strength exceeding 900 MPa and an elongation of at least 15%,that is, which is ensured to have the highest strength and a goodductility. To ensure a tensile strength exceeding 900 MPa, the Q valueis required to be at least 0.68. To ensure an elongation of at least15%, the Q value is required to be up to 1.00.

O, N and Fe are essential components as strengthening elements in thepresent invention, and exist without fail, in the alloy of the presentinvention, in content ranges satisfying the relationship with regard tothe Q value. For the reasons mentioned above, the N content is requiredto be up to 0.05% by weight, and the Fe content in accordance therewithis required to be at least 0.9% by weight. However, when the Fe contentbecomes excessive, solidification segregation becomes significant, andthe properties are deteriorated. Accordingly, the Fe content is definedto be up to 2.3% by weight.

In the present invention, part of Fe can be replaced with at least oneelement selected from Cr and Ni. Cr and Ni, as well as Fe, areβ-phase-stabilizing elements. These elements make grains fine, andcontribute to highly strengthening the titanium alloy. In this case, Qis defined by the following formula obtained by replacing the term [Fe]in the above-mentioned formula of Q with [Fe]+[Cr]+[Ni]:

    Q=[O]+2.77[N]+0.1{[Fe]+[Cr]+[Ni]}

wherein [O] is an oxygen content (% by weight), [N] is a nitrogencontent (% by weight), [Fe] is an iron content.(% by weight), [Cr] is achromium content (% by weight), and [Ni] is a nickel content (% byweight).

In this case also, the range of Q according to the present invention isfrom 0.9 to 2.3. To increase the strength and the ductilitysimultaneously, the Q value is required to be at least 0.9. When the Qvalue exceeds 2.3, solidification segregation becomes significant andthe properties are deteriorated as in case where Fe alone is addedwithout adding Cr and Ni.

However, when at least one element selected from Cr and Ni is added,addition of Cr or Ni in a large amount results in the formation of TiCr₂or Ti₂ Ni which are brittle compounds, and consequently the ductility islowered. To prevent the phenomenon, it is necessary that the contents ofCr and Ni should be defined to be each up to 0.25% by weight, and thatthe content of Fe should be defined to be at least 0.4% by weight,preferably at least 0.5% by weight.

The titanium alloy of the present invention usually contains C, H, Mo,Mn, Si, S, etc. as impurities as in the case of conventional puretitanium or a conventional titanium alloy. The contents are, however,each less than 0.05% by weight.

The titanium alloy of the present invention is usually prepared asdescribed below. Titanium is placed in a melting furnace, and arc meltedin vacuum or in an Ar atmosphere (VAR melting). In the presentinvention, a carbon steel and/or a stainless steel may be suppliedduring melting, whereby Fe and at least one element selected from Cr andNi can be added to Ti. Fe, Cr and Ni may be added in the total amount of0.9 to 2.3% by weight by the procedure mentioned above. Alternatively,these elements may be added by the above procedure in combination withanother addition procedure so that the addition amount falls in therange as mentioned above. Preferably, low cost scrap may also be used asa raw material.

Although there is no specific limitation on the addition raw materials,examples of the carbon steel and the stainless steel to be used areJIS-SS400, JIS-SUS430 (Fe--17Cr), JIS-SUS304 (Fe--18Cr--8Ni), JIS-SUS316(Fe--18Cr--8Ni), JIS-SUS316 (Fe--18Cr--8Ni--2Mo), and the like. AlthoughC, Mo, etc. are contained in these raw materials, the amounts of theseelements are trace compared with the contents of Fe, Cr and Ni. Theseelements belong to impurities the contents of which are each less than0.05% by weight.

In the present invention, Fe, Cr and Ni may also be added by other meansas described below.

That is, in refining titanium and producing sponge titanium by reductionwith Mg, i.e., by the Kroll process, a vessel made of a carbon steel orstainless steel is used. At least one element among Fe, Cr and Ni invadethe sponge titanium from the vessel, and sponge titanium containingthese elements is formed near the wall and the bottom of the vessel.Conventionally, the sponge titanium thus formed is separately collectedand used for other applications. In the present invention, however, itis used as part of or the whole of raw materials for the Fe, Cr and Niaddition. As a result, it becomes possible to produce the titanium alloyat low cost.

As described above, the present invention is capable of not onlyproviding a high strength, high ductility titanium alloy by adding O, N,Fe (and Cr and Ni) in defined amounts but also producing the titaniumalloy at low cost by the use of the low cost raw materials. Accordingly,the present invention is industrially extremely advantageous.

Furthermore, since the titanium alloy of the invention does not containAl as an alloying element, its hot workability is not lowered incontrast with conventional titanium alloys containing Al, and,therefore, its production is advantageous.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between a Q value and atensile strength.

FIG. 2 is a graph showing the relationship between a Q value and anelongation.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will be illustrated in more detail by makingreference to examples.

EXAMPLE 1

A high strength, high ductility titanium alloy having a tensile strengthof 700 to 900 MPa and an elongation of at least 20% was produced on thebasis of the first viewpoint of the present invention. In addition, inthe present example, "comparative example" signifies that it is outsidethe scope of the first viewpoint, and does not necessarily signify thatit is outside the scope of the second viewpoint.

(1) Cylindrical ingots having a diameter of 430 mm were prepared by VAR.The ingots were heated to 1,000° C., and forged to billets having adiameter of 100 mm. The billets were then heated to 850° C., and rolledto bars having a diameter of 12 mm. Moreover, the bars were annealed at700° C. for 1 hour. The production instance was designated "bar".

(2) Cylindrical ingots having a diameter of 430 mm were prepared by VAR.The ingots were heated to 1,000° C., and forged to slabs having athickness of 150 mm. The slabs were then heated to 850° C., and hotrolled to plates having a thickness of 4 mm. Moreover, the plates wereannealed at 700° C. for 1 hour. The production instance was designated"hot rolled plate".

(3) The hot rolled plates were descaled, and cold rolled to sheetshaving a thickness of 1.5 mm. The production instance was designated"cold rolled sheet".

The bars, the hot rolled plates and the cold rolled sheets produced bythe above procedures were subjected to tensile test (the following testpieces being adopted: bars: a test piece having a diameter of 12.5 mmand a gauge length of 50 mm; hot rolled plates and cold rolled sheets: aflat test piece having a width of 12.5 mm and a gauge length of 50 mm).Some of the test pieces were subjected to rotate-bending fatigue test(the non-failure strength at 10⁷ cycles being defined as fatiguestrength). The results are shown in Table 1 to Table 3.

Samples shown in Table 1 are those which contained chemical componentsrelated to the first viewpoint of the first aspect in the presentinvention. The addition of Fe was carried out with a pure metal, FeTi orFe₂ O₃ (iron oxide).

Samples shown in Table 2 are those which contained chemical componentsrelated to the first viewpoint of the second aspect in the presentinvention. The addition of Fe, Ni and Cr was conducted with pure metals,FeCr, FeNi, FeTi or Fe₂ O₃.

Table 3 shows examples of bars and hot rolled plates related to theproduction process of the present invention.

                                      TABLE 1                                     __________________________________________________________________________                    Tensile                                                                           Elong-                                                                            Fatigue                                               Test                                                                             Chemical component (wt. %)                                                                 strength                                                                          ation                                                                             strength                                              No.                                                                              O  N  Q* Fe  MPa %   MPa Remarks                                           __________________________________________________________________________    1  0.34                                                                             0.02                                                                             0.50                                                                             1.0 800 23.2                                                                              430 Bar, Ex., (typical,                                                           Fe being near lower limit)                        2  0.29                                                                             0.02                                                                             0.50                                                                             1.5 790 23.8                                                                              440 Bar, Ex, (typical)                                3  0.24                                                                             0.02                                                                             0.50                                                                             2.0 810 20.5                                                                              450 Bar, Ex., (typical,                                                           Fe being near lower limit)                        4  0.28                                                                             0.045                                                                            0.50                                                                             1.0 780 20.7                                                                              420 Bar, Ex., (N being                                                            near upper limit)                                 5  0.28                                                                             0.05                                                                             0.52                                                                             1.0 810 20.5                                                                              400 Bar, Ex., (N being                                                            upper limit)                                      6  0.23                                                                             0.06                                                                             0.50                                                                             1.0 820 16.6                                                                              310 Bar, Comp. Ex., (N                                                            exceeding upper limit -                                                        elongation, fatigue x)                           7  0.22                                                                             0.01                                                                             0.34                                                                             0.9 720 25.0                                                                              --  Bar, Ex., (Q being                                                            near lower limit)                                 8  0.20                                                                             0.01                                                                             0.32                                                                             0.9 680 25.5                                                                              --  Bar, Comp. Ex., (Q                                                            being low - strength being low x)                 9  0.39                                                                             0.02                                                                             0.60                                                                             1.5 880 20.7                                                                              --  Bar, Ex. (Q being                                                             near upper limit)                                 10 0.39                                                                             0.02                                                                             0.68                                                                             2.3 890 20.1                                                                              --  Bar, Ex., (Q and Fe                                                           being upper limit)                                11 0.42                                                                             0.03                                                                             0.70                                                                             2.0 950 16.3                                                                              --  Bar, Comp. Ex.,                                                               Q (being high ductility                                                       being low x)                                      12 0.27                                                                             0.01                                                                             0.38                                                                             0.8 680 26.0                                                                              --  Bar, Comp. Ex.,                                                               Fe (being low - strength being low x)             13 0.37                                                                             0.01                                                                             0.65                                                                             2.5 910 15.0                                                                              --  Bar, Comp. Ex.,                                                               Fe (being high - strength  being low x)           14 0.33                                                                             0.02                                                                             0.52                                                                             1.2 820 23.0                                                                              --  Hot rolled sheet, Ex.                             15 0.37                                                                             0.01                                                                             0.52                                                                             1.2 830 21.4                                                                              --  Hot rolled sheet, Ex.                             16 0.27                                                                             0.01                                                                             0.40                                                                             1.0 700 27.0                                                                              --  Cold rolled sheet, Ex.                            17 0.32                                                                             0.03                                                                             0.60                                                                             2.0 890 21.0                                                                              --  Cold rolled sheet, Ex.                            __________________________________________________________________________     Note:                                                                         *Q = an oxygen equivalent value = [O] + 2.77[N] + 0.1[Fe                      Ex. = Example = an example based on the first viewpoint of the first          aspect in the present invention                                          

                                      TABLE 2                                     __________________________________________________________________________                               Tensile                                                                           Elong-                                         Test                                                                             Chemical component (wt. %)                                                                            strength                                                                          ation                                          No.                                                                              O  N  Q* Fe Ni Cr Fe + Ni + Cr                                                                        MPa %   Remarks                                    __________________________________________________________________________    18 0.37                                                                             0.01                                                                             0.52                                                                             1.00                                                                             0.10                                                                             0.10                                                                             1.20  830 21.0                                                                              Hot rolled plate, Ex.                      19 0.37                                                                             0.01                                                                             0.50                                                                             1.00                                                                             0.25                                                                             0.25                                                                             1.50  870 20.5                                                                              Hot rolled plate, Ex., Ni and Cr                                              being                                                                         upper limit                                20 0.34                                                                             0.02                                                                             0.50                                                                             0.80                                                                             0.10                                                                             0.10                                                                             1.00  800 23.5                                                                              Hot rolled plate, Ex., Fe + Cr + Ni                                           being                                                                         near lower limit                           21 0.34                                                                             0.02                                                                             0.49                                                                             0.70                                                                             0.10                                                                             0.10                                                                             0.90  790 22.6                                                                              Hot rolled plate, Ex., Fe + Cr + Ni                                           being                                                                         lower limit                                22 0.27                                                                             0.01                                                                             0.39                                                                             0.55                                                                             0.15                                                                             0.15                                                                             0.85  680 27.0                                                                              Hot rolled plate, Comp. Ex., Fe + Cr +                                        Ni                                                                            being up to lower limit, strength                                             being                                                                         low x                                      23 0.32                                                                             0.03                                                                             0.60                                                                             1.70                                                                             0.15                                                                             0.15                                                                             2.00  880 22.3                                                                              Hot rolled plate, Ex., Fe + Cr + Ni                                           being                                                                         near upper limit                           24 0.32                                                                             0.03                                                                             0.63                                                                             2.00                                                                             0.15                                                                             0.15                                                                             2.30  890 20.6                                                                              Hot rolled plate, Ex., Fe + Cr + Ni                                           being                                                                         upper limit                                25 0.37                                                                             0.01                                                                             0.65                                                                             2.20                                                                             0.15                                                                             0.15                                                                             2.50  910 14.6                                                                              Hot rolled plate, Comp. Ex., Fe + Cr +                                        Ni                                                                            exceeding upper limit, ductility                                              being                                                                         low                                        26 0.29                                                                             0.02                                                                             0.49                                                                             1.00                                                                             0.30                                                                             0.05                                                                             1.35  770 18.2                                                                              Hot rolled plate, Comp. Ex., Ni being                                         excessive x                                27 0.29                                                                             0.02                                                                             0.44                                                                             0.45                                                                             0.30                                                                             0.15                                                                             0.90  730 16.3                                                                              Hot rolled plate, Comp. Ex., Ni being                                         excessive x, (Fe being relatively                                             insufficient)                              28 0.29                                                                             0.02                                                                             0.49                                                                             1.0                                                                              0.10                                                                             0.30                                                                             1.40  760 17.5                                                                              Hot rolled plate, Comp. Ex., Cr being                                         excessive x                                __________________________________________________________________________     Note:                                                                         *Q = an oxygen equivalent value = [O] + 2.77[N] + 0.1{[Fe] + [Cr] + [Ni]      Ex. = Example = an example based on the first viewpoint of the second         aspect in the present invention                                          

                                      TABLE 3                                     __________________________________________________________________________                               Tensile                                                                           Elong-                                         Test                                                                             Chemical component (wt. %)                                                                            strength                                                                          ation                                          No.                                                                              O  N  Q* Fe Ni Cr Fe + Ni + Cr                                                                        MPa %   Remarks                                    __________________________________________________________________________    29 0.34                                                                             0.02                                                                             0.50                                                                             0.9                                                                              0  0.18                                                                             1.08  810 22.7                                                                              Bar, Ex.                                                                      Ni and Cr being added from scrap of                                           SUS430,                                                                       Fe being further added from FeTi           30 0.37                                                                             0.01                                                                             0.52                                                                             1.0                                                                              0.07                                                                             0.15                                                                             1.22  820 22.1                                                                              Hot rolled plate, Ex.                                                         Ni and Cr being added from scrap of                                           SUS304,                                                                       Fe being further added from FeTi           31 0.37                                                                             0.01                                                                             0.52                                                                             1.0                                                                              0.08                                                                             0.16                                                                             1.24  840 21.5                                                                              Hot rolled plate, Ex.                                                         Ni and Cr being added from scrap of                                           SUS316,                                                                       Fe being further added from FeTi (**)      32 0.34                                                                             0.02                                                                             0.50                                                                             1.0                                                                              0  0  1.0   790 23.1                                                                              Bar, Ex.                                                                      entire Fe being added from SS400                                              (***)                                      33 0.27                                                                             0.02                                                                             0.52                                                                             0.9                                                                              0.1                                                                              0.2                                                                              1.2   810 21.7                                                                              Hot rolled plate, Ex.                                                         most of Fe, Ni and Cr being added                                             from                                                                          sponge titanium formed near                __________________________________________________________________________                                       vessel                                      Note:                                                                         *Q = an oxygen equivalent value = [O] + 2.77[N] + 0.1{[Fe] + [Ni] + [Cr]      ** Mo in an amount of 2% in SUS316 becoming impurities in an amount of        less than 0.02% in titanium alloy after melting.                              *** C in an amount of 0.1% in SS400 becoming impurities in an amount of       0.01% in titanium alloy after melting                                    

In Table 1, Test Nos. 1 to 5, 7, 9 and 10 (bars), and Test Nos. 14 to 17(hot rolled plates or cold rolled sheets) are examples based on thefirst viewpoint of the first aspect in the present invention. Thefeatures of each of the examples are described in the corresponding rowin the remarks column. The designation "typical" signifies that theexample is a typical one in the defined range.

Test No. 6 is a comparative example of a bar which had a low elongationand a low fatigue strength due to a high nitrogen content and which wasnot in the defined range. Test No. 8 is a comparative example of a barwhich had a low Q value (oxygen equivalent value: [O]+2.77[N]+0.1[Fe]).It is evident from the comparison of Test No. 8 with Test No. 7 thatsince Q in Test No. 8 was slightly outside the lower limit of thedefined range, the bar did not attain a tensile strength of 700 MPa.Test No. 11 is a comparative example of a bar which had a high Q valuedue to the high oxygen content. It is evident from the comparison ofTest No. 11 with Test No. 10 that since Q in Test No. 11 was slightlyoutside the upper limit of the defined range, the bar had a high tensilestrength and a low elongation. Test No. 12 is a comparative example of abar which did not attain a tensile strength in the defined range due toa low Fe content. Moreover, Test No. 13 is a comparative example of abar which had a solidification segregation, a high tensile strength anda considerably low elongation due to a high Fe content.

It can be seen from the above that a titanium alloy within the scope ofthe first viewpoint in the first aspect of the present invention has atensile strength of 700 to 900 MPa and an elongation of at least 20%.

In Table 2, Test Nos. 18 to 21, 23 and 24 are examples related to hotrolled plates and cold rolled sheets based on the first viewpoint of thesecond aspect in the invention, and the features of each of the examplesare described in the corresponding row in the remarks column.

Test No. 22 is a comparative example of a hot rolled plate which had alow content of Fe+Ni+Cr, and which had consequently a tensile strengthnot reaching the defined range. Test No. 25 is a comparative example ofa cold rolled sheet which had a large content of Fe+Ni+Cr and asolidification segregation, and which had consequently a tensilestrength exceeding the defined range and a considerably loweredelongation. Test No. 26 is a comparative example of a hot rolled platewhich had an excessive content of Ni and an insufficient elongation.Test No. 27 is a comparative example of a hot rolled plate which had aninsufficient content of Fe and an excessive content of Ni and a loweredelongation. Test No. 28 is a comparative example of a hot rolled platewhich had an excessive content of Cr and a lowered elongation. It can beseen from the above that a titanium alloy in the range of the firstviewpoint in the second aspect of the invention has a tensile strengthof 700 to 900 MPa and an elongation of at least 20%.

In Table 3, Test No. 29 is an example of a bar which was prepared withscrap SUS430 as a Cr source and FeTi as an Fe source during VAR meltingto have predetermined chemical components. Test No. 30 is an example ofa hot rolled plate which was prepared with scrap SUS304 as an Ni and Crsource and FeTi as an Fe source to have predetermined chemicalcomponents. Test No. 31 is an example of a hot rolled plate which wasprepared with scrap SUS316 as an Ni and Cr source and FeTi as an Fesource to have predetermined chemical components.

Test No. 32 is an example of a bar which was prepared with scrap ofSS400 to have predetermined chemical components.

Furthermore, Test No. 33 is an example of a hot rolled plate which wasprepared with cutout sponge titanium containing Fe, Ni and Cr which hadinvaded from a stainless steel vessel in the step of producing spongetitanium, to have predetermined chemical components.

The contents of the chemical components of the samples are as shown inTable 3. Moreover, each of the samples had a tensile strength of atleast 700 MPa and an elongation of at least 20%, namely in the range ofthe first viewpoint in the first and the second aspect of the invention,and exhibited excellent properties.

EXAMPLE 2

A high strength, high ductility titanium alloy having a tensile strengthof at least 850 MPa and an elongation of at least 15% was produced onthe basis of the second viewpoint in the present invention. In addition,in the present example, "comparative example" signifies that it isoutside the scope of the second viewpoint, and does not necessarilysignify that it is outside the scope of the first viewpoint.

(1) Cylindrical ingots having a diameter of 430 mm were prepared by VAR.The ingots were heated to 1,000° C., and forged to billets having adiameter of 100 mm. The billets were then heated to 850° C., and rolledto bars having a diameter of 12 mm. Moreover, the bars were annealed at700° C. for 1 hour. The production instance was designated "bar".

(2) Cylindrical ingots having a diameter of 430 mm were prepared by VAR.The ingots were heated to 1,000° C., and forged to slabs having athickness of 150 mm. The slabs were then heated to 850° C., and hotrolled to plates having a thickness of 4 mm. Moreover, the plates wereannealed at 700° C. for 1 hour. The production instance was designated"hot rolled plate".

(3) The hot rolled plates were descaled, and cold rolled to sheetshaving a thickness of 1.5 mm. The production instance was designated"cold rolled sheet".

The bars, the hot rolled plates and the cold rolled sheets produced bythe above procedures were subjected to tensile test (the following testpieces being adopted: bars: a test piece having a diameter of 12.5 mmand a gauge length of 50 mm; hot rolled plates and cold rolled sheets: aflat test piece having a width of 12.5 mm and a gauge length of 50 mm).Part of them were subjected to rotate-bending fatigue test (thenon-failure strength at 10⁷ cycles being defined as fatigue strength).The results are shown in Table 4 to Table 6.

Samples shown in Table 4 are those which contained chemical componentsrelated to the first aspect of the present invention. The addition of Fewas carried out with pure metal, FeTi or Fe₂ O₃ (iron oxide)

Samples shown in Table 5 are those which contained chemical componentsrelated to the second aspect of the present invention. The addition ofFe, Ni and Cr was carried out with pure metals, FeCr, FeNi, FeTi or Fe₂O₃.

Table 6 shows examples of bars and hot rolled plates related to theproduction process of the present invention.

                                      TABLE 4                                     __________________________________________________________________________                            Rotate-                                                                       bending                                                               Tensile                                                                           Elong-                                                                            fatigue                                               Test                                                                             Chemical component (wt. %)                                                                 strength                                                                          ation                                                                             strength                                              No.                                                                              O  N  Fe Q*  MPa %   MPa Remarks                                           __________________________________________________________________________    1  0.37                                                                             0.02                                                                             1.20                                                                             0.55                                                                              860 23.0                                                                              --  Hot rolled plate, Ex.                             2  0.57                                                                             0.02                                                                             1.20                                                                             0.75                                                                              990 20.5                                                                              --  Hot rolled plate, Ex.                             3  0.77                                                                             0.04                                                                             0.70                                                                             0.95                                                                              1100                                                                              14.0                                                                              --  Hot rolled plate,                                                             Conventional Ex.                                  4  0.75                                                                             0.04                                                                             0.90                                                                             0.95                                                                              1130                                                                              15.8                                                                              --  Hot rolled plate,                                                             Ex., Fe being lower limit                         5  0.72                                                                             0.04                                                                             1.20                                                                             0.95                                                                              1150                                                                              16.5                                                                              --  Hot rolled plate, Ex.                             6  0.27                                                                             0.02                                                                             1.20                                                                             0.45                                                                              820 23.5                                                                              --  Hot rolled plate,                                                             Comp. Ex., Q being overly low -                                               strength being low                                7  0.82                                                                             0.04                                                                             1.20                                                                             1.05                                                                              1190                                                                              9.5 --  Hot rolled plate,                                                             Comp. Ex., Q being excessive -                                                ductility being low                               8  0.53                                                                             0.045                                                                            1.00                                                                             0.75                                                                              1010                                                                              19.2                                                                              540 Bar, Ex., N being                                                             near upper limit                                  9  0.53                                                                             0.05                                                                             1.00                                                                             0.77                                                                              1040                                                                              18.5                                                                              550 Bar, Ex., N being                                                             upper limit                                       10 0.50                                                                             0.055                                                                            1.00                                                                             0.75                                                                              1020                                                                              11.0                                                                              360 Bar, Comp. Ex., N                                                             being excessive - ductility and                                               fatigue strength being low                        11 0.55                                                                             0.045                                                                            0.75                                                                             0.75                                                                              1010                                                                              12.5                                                                              390 Bar, Comp. Ex., Fe                                                            being overly low - ductility and                                              fatigue strength being low                        12 0.49                                                                             0.02                                                                             2.00                                                                             0.75                                                                              970 20.1                                                                              520 Bar, Ex., Fe                                                                  being near upper limit                            13 0.49                                                                             0.02                                                                             2.3                                                                              0.78                                                                              990 19.5                                                                              510 Bar, Ex., Fe                                                                  being upper limit                                 14 0.44                                                                             0.02                                                                             2.50                                                                             0.75                                                                              980 11.5                                                                              360 Bar, Comp. Ex., Fe                                                            being excessive - ductility and                                               fatigue strength being low                        15 0.40                                                                             0.01                                                                             1.20                                                                             0.55                                                                              870 22.5                                                                              --  Cold rolled sheet, Ex.                            16 0.50                                                                             0.01                                                                             1.20                                                                             0.65                                                                              910 21.7                                                                              --  Cold rolled sheet, Ex.                            __________________________________________________________________________     Note:                                                                         *Q = [O] + 2.77[N] + 0.1[Fe                                              

                                      TABLE 5                                     __________________________________________________________________________                               Tensile                                                                           Elong-                                         Test                                                                             Chemical component (wt. %)                                                                            strength                                                                          ation                                          No.                                                                              O  N  Fe Ni Cr Fe + Ni + Cr                                                                        Q* MPa %   Remarks                                    __________________________________________________________________________    17 0.57                                                                             0.02                                                                             1.0                                                                              0.1                                                                              0.1                                                                              1.2                                                                              0.75  980 20.7                                                                              Hot rolled plate, Ex.                      18 0.60                                                                             0.02                                                                             0.6                                                                              0.15                                                                             0.1                                                                              0.9                                                                              0.75  980 19.8                                                                              Hot rolled plate, Ex.                                                         Fe + Ni + Cr being lower limit             19 0.60                                                                             0.02                                                                             0.6                                                                              0.25                                                                             0.25                                                                             1.1                                                                              0.77  990 18.8                                                                              Hot rolled plate, Ex.                                                         Ni and Cr being upper limit                20 0.61                                                                             0.02                                                                             0.65                                                                             0.10                                                                             0.05                                                                             0.80                                                                             0.75  930 12.5                                                                              Hot rolled plate, Comp. Ex., Fe + Ni +                                        Cr being                                                                      insufficient, strength and ductility                                          being                                                                         low                                        21 0.48                                                                             0.02                                                                             1.8                                                                              0.15                                                                             0.15                                                                             2.1                                                                              0.75  970 20.5                                                                              Hot rolled plate, Ex.                                                         Fe + Ni + Cr being near upper limit        22 0.48                                                                             0.02                                                                             2.0                                                                              0.15                                                                             0.15                                                                             2.3                                                                              0.77  980 20.2                                                                              Hot rolled plate, Ex.                                                         Fe + Ni + Cr being upper limit             23 0.43                                                                             0.02                                                                             2.15                                                                             0.17                                                                             0.18                                                                             2.5                                                                              0.75  960 11.5                                                                              Hot rolled plate, Comp. Ex., Fe + Ni +                                        Cr being                                                                      excessive - ductility being low            24 0.42                                                                             0.01                                                                             0.80                                                                             0.10                                                                             0.10                                                                             1.0                                                                              0.55  870 21.5                                                                              Cold rolled sheet, Ex.                     25 0.42                                                                             0.01                                                                             0.65                                                                             0.30                                                                             0.05                                                                             1.0                                                                              0.55  860 11.5                                                                              Cold rolled sheet, Comp. Ex., Ni                                              being                                                                         excessive - ductility being low            26 0.42                                                                             0.01                                                                             0.65                                                                             0.05                                                                             0.30                                                                             1.0                                                                              0.55  870 12.5                                                                              Cold rolled plate, Comp. Ex., Cr                                              being                                                                         excessive - ductility being                __________________________________________________________________________                                       low                                         Note:                                                                         *Q = [O] + 2.77[N] + 0.1{[Fe] + [Ni] + [Cr]                              

                                      TABLE 6                                     __________________________________________________________________________                               Tensile                                                                           Elong-                                         Test                                                                             Chemical component (wt. %)                                                                            strength                                                                          ation                                          No.                                                                              O  N  Fe Ni Cr Fe + Ni + Cr                                                                        Q* MPa %   Remarks                                    __________________________________________________________________________    27 0.56                                                                             0.03                                                                             0.9                                                                              0  0.18                                                                             1.08                                                                             0.75  970 21.0                                                                              Hot rolled plate, Ex.                                                         Ni and Cr being added from scrap of                                           SUS430,                                                                       Fe being further added from FeTi           28 0.54                                                                             0.03                                                                             1.0                                                                              0.07                                                                             0.22                                                                             1.29                                                                             0.75  990 20.8                                                                              Hot rolled plate, Ex.                                                         Ni and Cr being added from scrap of                                           SUS304,                                                                       Fe being further added from FeTi           29 0.53                                                                             0.03                                                                             1.0                                                                              0.08                                                                             0.24                                                                             1.32                                                                             0.75  990 21.1                                                                              Hot rolled plate, Ex.                                                         Ni and Cr being added from scrap of                                           SUS316,                                                                       Fe being further added from FeTi (**)      30 0.54                                                                             0.03                                                                             1.0                                                                              0  0  1.0                                                                              0.72  950 23.1                                                                              Hot rolled plate, Ex.                                                         entire Fe being added from SS400                                              (***)                                      31 0.56                                                                             0.03                                                                             1.0                                                                              0.1                                                                              0.2                                                                              1.3                                                                              0.77  1010                                                                              18.7                                                                              Hot rolled plate, Ex.                                                         most of Fe, Ni and Cr being added                                             from                                                                          sponge titanium formed near                __________________________________________________________________________                                       vessel                                      Note:                                                                         *Q = [O] + 2.77[N] + 0.1{[Fe] + [Ni] + [Cr]                                   ** Mo in an amount of 2% in SUS316 becoming impurities in an amount of        less than 0.02% in titanium alloy after melting.                              *** C in an amount of 0.1% in SS400 becoming impurities in an amount of       0.01% in titanium alloy after melting                                    

In Table 4, Test Nos. 1, 2, 4 and 5 (hot rolled plates), Test Nos. 8, 9,12 and 13 (bars) and Test Nos. 15 and 16 (cold rolled sheets) areexamples based on the second viewpoint of the first aspect in thepresent invention. The features of each of the examples are described inthe corresponding row in the remarks column.

Test No. 3 is a conventional example of a hot rolled plate which had alow Fe content and a low elongation not reaching the defined range. TestNo. 6 is a comparative example of a hot rolled plate which had a lowvalue of Q (oxygen equivalent value: [O]+2.77[N]+0.1[Fe]) and aninsufficient tensile strength. It is evident from the comparison of TestNo. 6 with Test No. 1 that since Q in Test No. 6 was slightly outsidethe lower limit of the defined range, the hot rolled plate did notattain a tensile strength of 850 MPa. Test No. 7 is a comparativeexample of a hot rolled sheet which had a high Q value due to a highoxygen content. Although the hot rolled plate had a high tensilestrength, it had a considerably low elongation.

Test No. 10 is a comparative example of a bar which had a high nitrogencontent and a low elongation and a low fatigue strength. Test No. 11 isa comparative example of a bar which had a low Fe content and a lowelongation and a low fatigue strength. Moreover, Test No. 14 is acomparative example of a bar which had a solidification segregation anda low elongation and a low fatigue strength due to a high Fe content.

It can be seen from the above that a titanium alloy within the scope ofthe second viewpoint in the first aspect of the present invention has atensile strength of at least 850 MPa and an elongation of at least 15%.

In Table 5, Test Nos. 17 to 19, 21, 22 and 24 are examples related tohot rolled sheets and cold rolled sheets based on the second viewpointof the second aspect of the invention, and the features of each of theexamples are described in the corresponding row in the remarks column.

Test No. 20 is a comparative example of a hot rolled plate which had alow total content of Fe+Ni+Cr, and which consequently did not attain anelongation in the defined range. Test No. 23 is a comparative example ofa cold rolled plate which had a large content of Fe+Ni+Cr and asolidification segregation, and which had consequently a considerablylowered elongation. Test No. 25 is a comparative example of a coldrolled sheet which had an excessive content of Ni and an insufficientelongation. Test No. 26 is an example of a cold rolled sheet which hadan excessive content of Cr and an insufficient elongation. It can beseen from the results described above that a titanium alloy within thescope of the second viewpoint in the second aspect of the invention hasa tensile strength of at least 850 MPa and an elongation of at least15%.

In Table 6, Test No. 27 is an example of a bar which was prepared withscrap of SUS430 as an Fe and Cr source and FeTi as an Fe source duringVAR melting to have predetermined chemical components. Test No. 28 is anexample of a hot rolled plate which was prepared with scrap SUS304 as anFe, Ni and Cr source and FeTi as an Fe source to have predeterminedchemical components. Test No. 29 is an example of a hot rolled platewhich was prepared with scrap of SUS316 as an Fe, Ni and Cr source andFeTi as an Fe source to have predetermined chemical components.

Test No. 30 is an example of a bar which was prepared with scrap ofSUS400 as an Fe source to have predetermined chemical components.

Furthermore, Test No. 31 is an example of a hot rolled plate which wasprepared with cutout sponge titanium containing Fe, Ni and Cr which hadinvaded from a stainless steel vessel in the step of producing spongetitanium, to have predetermined chemical components.

The contents of the chemical components of the samples are as shown inTable 6. Moreover, each of the samples had a tensile strength of atleast 850 MPa and an elongation of at least 15%, namely in the range ofthe second viewpoint of the first and the second aspect in theinvention, and exhibited excellent properties.

EXAMPLE 3

A high strength, high ductility titanium alloy having a tensile strengthof at least 850 MPa and an elongation of at least 15% was produced onthe basis of the second viewpoint of the present invention. In addition,"a comparative example" in the present invention signifies that it isoutside the scope of the second viewpoint and does not necessarilysignify that it is outside the scope of the first viewpoint.

Samples containing 1.5% by weight of Fe (examples) or 0.7% by weight ofFe (comparative examples) and having Q values as shown in Table 7 wereprepared as described below. Cylindrical ingots having a diameter of 100mm were melted by plasma arc melting. The ingots were heated to 1,000°C., and forged to slabs having a thickness of 80 mm. The slabs were thenheated to 850° C., and hot rolled to hot rolled plates having athickness of 4 mm. The hot rolled plates were annealed at 700° C. for 1hr. The samples thus obtained were subjected to the tensile testdescribed in Example 1. The results thus obtained are plotted and shownin FIGS. 1 and 2.

It is understood from the figures that the alloys containing 1.5% of Fein the present invention (denoted by the mark ο) exhibit improvedtensile strength and elongation from a Q value of at least 0.5, incomparison with conventional alloys (0.7% Fe, denoted by the mark •).The improvement becomes particularly significant when Q=0.68-1.00.

                  TABLE 7                                                         ______________________________________                                                       Tensile                                                                       strength  Elongation                                           Chemical component (wt. %)                                                                   (MPa)**   (%)**                                                Fe   O      N      Q*    min. max. min. max. Remarks                          ______________________________________                                        0.7  0.08   0.01   0.18  590  610  29.3 31.4 Comp. Ex                         0.7  0.17   0.01   0.27  650  670  26.5 28.5 Comp. Ex                         0.7  0.25   0.01   0.35  700  730  25.5 27.0 Comp. Ex                         0.7  0.33   0.02   0.46  770  790  22.7 24.1 Comp. Ex                         0.7  0.40   0.02   0.53  790  820  20.6 22.5 Comp. Ex                         0.7  0.48   0.02   0.61  840  860  19.2 21.4 Comp. Ex                         0.7  0.56   0.02   0.69  890  910  18.3 19.5 Comp. Ex                         0.7  0.60   0.03   0.75  920  950  16.7 18.6 Comp. Ex                         0.7  0.71   0.03   0.86  1000 1030 14.0 16.6 Comp. Ex                         0.7  0.77   0.04   0.95  1050 1080 12.5 15.0 Comp. Ex                         0.7  0.83   0.04   1.01  1070 1110 10.5 12.8 Comp. Ex                         0.7  0.90   0.04   1.08  1120 1170 9.2  11.5 Comp. Ex                         0.7  0.95   0.04   1.13  1150 1190 6.1  9.1  Comp. Ex                         1.5  0.04   0.005  0.20  600  620  28.0 30.1 Comp. Ex                         1.5  0.07   0.01   0.25  640  660  26.3 29.0 Comp. Ex                         1.5  0.19   0.01   0.37  720  740  25.2 27.3 Comp. Ex                         1.5  0.23   0.02   0.44  790  810  22.8 24.4 Comp. Ex                         1.5  0.34   0.02   0.55  860  890  21.3 23.2 Ex.                              1.5  0.39   0.02   0.60  890  920  20.0 22.3 Ex.                              1.5  0.45   0.03   0.68  940  960  19.7 21.8 Ex.                              1.5  0.49   0.03   0.72  1000 1030 19.5 21.3 Ex.                              1.5  0.62   0.04   0.88  1110 1140 18.6 20.2 Ex.                              1.5  0.67   0.04   0.93  1180 1210 17.1 19.3 Ex.                              1.5  0.73   0.04   0.99  1200 1250 16.0 18.5 Ex.                              1.5  0.79   0.04   1.05  1250 1280 10.5 15.0 Comp. Ex                         1.5  0.89   0.04   1.15  1270 1330 8.5  10.5 Comp. Ex                         ______________________________________                                         Note:                                                                         *Q = [O] + 2.77[N] + 0.1                                                      **the maximum value and the minimum value obtained from 5 samples        

INDUSTRIAL APPLICABILITY

As explained above, the present invention provides a high strength, highductility titanium alloy which was prepared by increasing an Fe contentas a strengthening element while the N content is decreased, adjustingthe contents of strengthening elements O, N and Fe, or those ofstrengthening elements O, N, Fe, and Cr and Ni (Cr and Ni replacing partof Fe) through adjusting an oxygen equivalent value Q. Moreover,according to the present invention, the strengthening elements mentionedabove can be supplied from low cost raw materials, and, therefore, thetitanium alloy may be produced at low cost. Accordingly, the presentinvention is extremely advantageous from an industrial standpoint.

We claim:
 1. A high strength, high ductility titanium allow free of Al,V and Mo and comprising O, N and Fe as strengthening elements and thebalance substantially Ti, the contents of the strengthening elementssatisfying the following relationships (1) to (3):(1) from 0.9 to 2.3%by weight of Fe, (2) up to 0.05% by weight of N, and (3) an oxygenequivalent value Q, which is defined by the formula mentioned below, of0.40 to 1.00

    Q=[O]+2.77[N]+0.1[Fe]

wherein [O] is the oxygen content (% by weight), [N] is the nitrogencontent (% by weight) and [Fe] is the iron content (% by weight), thetitanium alloy having a tensile strength of at least 700 MPa and anelongation of at least 15%.
 2. A high strength, high ductility titaniumalloy free of Al, V and Mo and comprising O, N, Fe and at least oneelement selected from Cr and Ni as strengthening elements and thebalance substantially Ti, the contents of the strengthening elementssatisfying the following relationships (1) to (6):(1) from 0.9 to 2.3%by weight of the total amount of Fe, Cr and Ni, (2) at least 0.4% byweight of Fe, (3) up to 0.25% by weight of Cr, (4) up to 0.25% by weightof Ni, (5) up to 0.05% by weight of N, and (6) an oxygen equivalentvalue Q, which is defined by the formula mentioned below, of 0.40 to1.00

    Q=[O]+2.77[N]+0.1{[Fe]+[Cr]+[Ni]}

wherein [O] is the oxygen content (% by weight), [N] is the nitrogencontent (% by weight), [Fe] is the iron content (% by weight), [Cr] isthe Cr content (% by weight) and [Ni] is the Ni content (% by weight),the titanium alloy having a tensile strength of at least 700 MPa and anelongation of at least 15%.
 3. The high strength, high ductilitytitanium alloy as claimed in claim 1, wherein the oxygen equivalentvalue Q is from 0.40 to 0.68, and the titanium alloy has a tensilestrength of 700 to 900 MPa and an elongation of at least 20%.
 4. Thehigh strength, high ductility titanium alloy as claimed in claim 1,wherein the oxygen equivalent value Q is from 0.50 to 1.00, and thetitanium alloy has a tensile strength of at least 850 MPa and anelongation of at least 15%.
 5. The high strength, high ductilitytitanium alloy as claimed in claim 4, wherein the oxygen equivalentvalue Q is from greater than 0.68 to 1.00, and the titanium alloy has atensile strength exceeding 900 MPa.
 6. The high strength, high ductilitytitanium alloy as claimed in claim 2, wherein the oxygen equivalentvalue Q is from 0.40 to 0.68, and the titanium alloy has a tensilestrength of 700 to 900 MPa and an elongation of at least 20%.
 7. Thehigh strength, high ductility titanium alloy as claimed in claim 2,wherein the oxygen equivalent value Q is from 0.50 to 1.00, and thetitanium alloy has a tensile strength of at least 850 MPa and anelongation of at least 15%.
 8. The high strength, high ductilitytitanium alloy as claimed in claim 7, wherein the oxygen equivalentvalue Q is from greater than 0.68 to 1.00, and the titanium alloy has atensile strength exceeding 900 MPa.
 9. A process for producing a highstrength, high ductility titanium alloy comprising O, N and Fe asstrengthening elements and the balance substantially Ti, the contents ofthe strengthening elements satisfying the following relationships (1) to(3):(1) from 0.9 to 2.3% by weight of Fe, (2) up to 0.05% by weight ofN, and (3) an oxygen equivalent value Q, which is defined by the formulamentioned below, of 0.40 to 1.00

    Q=[O]+2.77[N]+0.1[Fe]

wherein [O] is the oxygen content (% by weight), [N] is the nitrogencontent (% by weight) and [Fe] is the iron content (% by weight), thetitanium alloy having a tensile strength ot at least 700 MPa and anelongation of at least 15%, the process comprising charging at least onesteel selected from carbon steels and stainless steels to a melt ottitanium during the production of said titanium alloy, so that at leastpart of Fe as the strengthening element is introduced from said steel tosaid titanium alloy.
 10. A process for producing a high strength, highductility titanium alloy comprising O, N, Fe and at least one elementselected from Cr and Ni as strengthening elements and the balancesubstantially Ti, the contents of the strengthening elements satisfyingthe following relationships (1) to (6):(1) from 0.9 to 2.3% by weight ofthe total amount of Fe, Cr and Ni, (2) at least 0.4% by weight of Fe,(3) up to 0.25% by weight of Cr, (4) up to 0.25% by weight of Ni, (5) upto 0.05% by weight of N, and (6) an oxygen equivalent value Q, which isdefined by the formula mentioned below, of 0.40 to 1.00

    Q=[O]+2.77[N]+0.1{[Fe]+[Cr]+[Ni]}

wherein [O] is the oxygen content (% by weight), [N] is the nitrogencontent (% by weight), [Fe] is the iron content (% by weight), [Cr] isthe Cr content (% by weight) and [Ni] is the Ni content (% by weight),the titanium alloy having a tensile strength of at least 700 MPa and anelongation of at least 15%, the process comprising charging at least onesteel selected from carbon steels and stainless steels to a melt oftitanium during the production of said titanium alloy, so that at leastpart of Fe, Cr and Ni as the strengthening elements is introduced fromsaid steel to said titanium alloy.
 11. A process for producing a highstrength, high ductility titanium alloy comprising O, N and Fe asstrengthening elements and the balance substantially Ti, the contents ofthe strengthening elements satisfying the following relationships (1) to(3):(1) from 0.9 to 2.3% by weight of Fe, (2) up to 0.05% by weight ofN, and (3) an oxygen equivalent value Q, which is defined by the formulamentioned below, of 0.40 to 1.00

    Q=[O]+2.77[N]+0.1[Fe]

wherein [O] is the oxygen content (% by weight), [N] is the nitrogencontent (% by weight) and [Fe] is the iron content (% by weight), thetitanium alloy having a tensile strength of at least 700 MPa and anelongation of at least 15%, the process comprising: producing spongetitanium in a vessel made from a steel containing Fe, whereby Fe fromwalls or bottom or both of said vessel is transferred into said spongetitanium, and then charging said sponge titanium containing said Fe asat least part of the source for Fe as a strengthening element in theproduction of the titanium alloy.
 12. A process for producing a highstrength, high ductility titanium alloy comprising O, N, Fe and at leastone element selected from Cr and Ni as strengthening elements and thebalance substantially Ti, the contents of the strengthening elementssatisfying the following relationships (1) to (6):(1) from 0.9 to 2.3%by weight of the total amount of Fe, Cr and Ni, (2) at least 0.4% byweight of Fe, (3) up to 0.25% by weight of Cr, (4) up to 0.25% by weightof Ni, (5) up to 0.05% by weight of N, and (6) an oxygen equivalentvalue Q, which is defined by the formula mentioned below, of 0.40 to1.00

    Q=[O]+2.77[N]+0.1{[Fe]+[Cr]+[Ni]}

wherein [O] is the oxygen content (% by weight), [N] is the nitrogencontent (% by weight), [Fe] is the iron content (% by weight), [Cr] isthe Cr content (% by weight) and [Ni] is the Ni content (% by weight),the titanium alloy having a tensile strength of at least 700 MPa and anelongation of at least 15%, the process comprising: producing spongetitanium in a vessel made from a steel containing at least one elementselected from Fe, Cr and Ni whereby said at least one element from wallsor bottom or both of said vessel is transferred into said spongetitanium, and then charging said sponge titanium containing said atleast one element as at least part of the source for the at least oneelement selected from Fe, Cr and Ni as the strengthening element in theproduction of the titanium alloy.